Friction stir welding is a process in which a rotating, round, cylindrical tool with an axially extending pin or protrusion may be used to form a weld between two or more layers of assembled metal workpieces including sheet, extrusions, and castings. The rotating centered pin and adjoining shoulder portion, which defines the maximum tool-workpiece contact region or tool ‘footprint’, are plunged into a surface of one sheet. The affected sheet metal is softened and plasticized by the frictional heat, and the tool is driven through the first sheet, through any intervening sheet layer, and into the bottom sheet of the assembly, mixing softened metal from each sheet layer into a bonding weld mass. The tool may then be retracted to form a friction stir spot weld or progressively advanced through the sheet layers along a proposed joint line to form a continuous linear seam weld. The assembly of sheet metal layers is often supported against the welding force of the rotating tool by an anvil aligned in opposition with the axis of the tool.
The inventors herein recognize that one potential application for continuous friction stir welding is the fabrication of automobile closure panel assemblies like doors, hoods or decklids. Such assemblies typically comprise an outer panel and a complementary inner panel. The panels may be three or four feet in major directions and cumbersome to handle. Both inner and outer panels may display complex curvature depending on the design of the vehicle and they may be further shaped to accommodate a window, or wiring, or closure mechanisms between them. Typically, the panels have like-shaped peripheral edges at which they are bonded or welded. Sometimes edges of the outer panel are folded over edges of the inner panel in a hem attachment. And welding or other bonding is accomplished through the layers of the hem.
The inventors recognize that friction stir welding might be used to form a continuous seam weld around the peripheral surfaces of assembled vehicle panels fabricated of about 1 millimeter thick sheet. These panels, due to the curvature of the panels and the direction-changing path of their bonding surfaces, present difficulties to present friction stir tool and anvil designs. In this case, conventional practice would call for very large and complex anvils to fully support the friction stir tool over the entire length of the joint path.
The geometry of such a joint, requiring as it does that the tool traverse a long joint with a continuous anvil backup and accommodate joint path variations both in and out of the plane of the joint, requires different tooling—tooling that better accommodates direction-changing movement of the sheet layers between a weld tool and anvil. Also, the anvil will support the welding process by contact with a portion of the vehicle panel's exterior surface, a surface readily viewed by a customer and more often known as a ‘show surface’. Any blemish or appearance defect in such a ‘show surface’ may be removed in a ‘metal finishing’ operation prior to painting the vehicle but this process incurs additional expense. Thus, a further requirement on such a compact friction stir anvil suitable for use on show surfaces is that the anvil support such ‘show surfaces’ without imparting blemishes to the ‘show panel’.
Another potential application for continuous or semi-continuous friction stir welding is in the fabrication of vehicle structures. Such structures typically comprise multiple panels, which may be up to 4 millimeters thick, and heavier gauge reinforcements. These panels are joined together, conventionally by spot welding, often supplemented by adhesive bonding. Here, access of friction stir weld tooling to the weld zone, particularly access by the anvil, over a continuous length of at least a portion of the overall joint is hindered by a generally complex joint geometry and the number of components involved. However the installation of continuous welds or semi-continuous stitch welds within a vehicle structure could improve static strength, fatigue strength, and overall stiffness and may eliminate adhesives.
There is, therefore, need for a compact movable anvil capable of moving in conjunction with the friction stir weld tool and relative to the workpiece to provide support to the workpiece as the tool and anvil traverse the length of a weld joint. A further desired characteristic of such a compact anvil is that it minimizes frictional interaction between the anvil and anvil-supported workpiece surface when under normal load to enable relative motion between the workpieces and friction stir welder under greatly reduced lateral forces.